This invention relates to the field of controlling the extrusion gap between a pair of die lips for forming sheet material, as an example.
It has been well known to move one or more die lips with respect to the another in response to the measurements taken by a downstream guage or thickness measuring apparatus, to control the thickness of the extrudate. A widely used arrangement includes the selective heating and/or cooling of extensible die bolts, one end of which is anchored and the other end of which causes the flexure of one die lip with respect to another die lip. Such arrangements are shown in Nissel U.S. Pat. No. 3,940,221, Cloeren et al. U.S. Pat. No. 5,020,984 and in Hatori et al. U.S. Pat. No. 5,051,082, as examples.
A particular disadvantage of the apparatus as shown, for example, in the above referenced patents is the fact that the hinged or flexible die lip, when biased or flexed differentially, such as at a series of longitudinal positions along the die, causes a distortion in the gap and therefore causes a non-uniformity with respect to the surface of a deckle rod which may be positioned within the die. Such distortions and lack of uniformities can cause leakage past a deckle rod.
External sliding as adjustable die lips arranged as opposed pairs, have been used in combination with die bodies. These die lips provide opposed extrusion land surfaces which are in alignment with an extrusion slot in the die body, but are separate from the die body itself, so that the die lips may be mechanically adjusted by relative sliding movement along beveled lower surfaces of the die body, to open or close an extrusion gap. Typical externally mounted and adjustable die lips, in combination with a deckle rod system, are shown in Maejima U.S. Pat. Nos. 4,248,578 and 4,659,302. In these patents, an internal seal at the extrusion gap is formed with the sloping inside surfaces of the adjustable die lips, such as rod 6 and die lip 17 of patent U.S. Pat. No. 4,248,579. An advantage of this known type of system is that adjustment of the extrusion gap does not cause a change in geometry of the seating surfaces of the deckle rod on the sloping inside surfaces of the die lips. To Applicant's knowledge, no prior satisfactory arrangement has been proposed by which a solid sliding external die lip of the kind shown in the Maejima patents has been actuated by thermal actuation.
A further difficulty and limitation inherent to many of the prior thermal bolt actuators resides is the fact that the die slot or gap is closed by the application of heat which expands the die bolt. At the same time, the heat is not effectively isolated from the die body and the heat causes a decrease in the polymer viscosity within the extrusion slot. This decrease in polymer viscosity runs counter to the decrease in slot width, by tending to increase the rate of polymer flow. Accordingly, accurate control of guage thickness can become difficult to achieve.
U.S. Pat. No. 5,051,082 describes a system in FIG. 11 of that patent in which, working through a flexible or bendable die hinged die lip, the gap is increased with increasing die bolt temperature and the gap is decreased with decreasing die bolt temperature, in combination with an airflow arrangement for cooling the die bolt. The die bolt itself is formed with an aperture throughout a major portion of its length providing a passageway for the air to flow for cooling the die bolt. The cooling air is brought into the die bolt by an air feed tube which is common to all of the die bolts. This arrangement puts all of the die bolt cooling systems in series with each other and at a constant cooling bias, with the result that any one die bolt could be heated or cooled at a time when it needs to be heated. Further, heat migration, which is undesirable for control purposes, is intentionally allowed to flow into the die body so that the viscosity of the molten resin is changed at the same time that the die lip is undergoing adjustment.